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How to perform a monohybrid cross 2.1.10.1 Overview Monohybrid crosses show how certain traits are passed down from parents to their offspring. By completing a Punnett square, we can predict the genotypes and phenotypes of offspring resulting from a monohybrid cross, for both autosomal and sex-linked traits. Theory details In chapter 7, you learned about alleles and the notation we use to describe genotypes. A monohybrid cross explores how alleles are passed down from parents to their children. Autosomal complete dominance Punnett square A cross between two heterozygotes results in three possible genotypes in the offspring. Figure 1 demonstrates the allelic inheritance of a monohybrid cross between two individuals heterozygous for the widow’s peak gene (Ww), where W = widow’s peak allele and w = straight hairline allele. Each parent passes on one of the two possible alleles via their gametes. Since half of the mother’s gametes contain the dominant allele, W, she has a 50% chance of passing on the dominant allele, W. Similarly, she has a 50% chance of passing on the recessive allele, w. The same odds apply for the father since he is the same genotype as the mother. From here, we can predict the possible genotype.

Monohybrid test crosses 2.1.10.2 Overview For traits that operate via complete dominance, an individual that expresses the dominant phenotype could either be homozygous dominant and heterozygous. To determine the genotype of such an individual, we can use a monohybrid test cross. Theory details A monohybrid test cross is performed by crossing an individual with an unknown genotype with a homozygous recessive individual. Then, by observing the phenotypes of all the offspring you can determine whether the unknown parent is homozygous dominant or heterozygous. In a sense, it is the reverse of a regular monohybrid cross as we are using the phenotypes of the offspring to determine a parent’s genotype. In Figure 4, a black sheep (BB or Bb) and a white sheep (bb) are shown. If the two parental sheep were to have at least one white offspring you could instantly determine the genotype of the black sheep parent to be Bb, whereas if all offspring were black you could conclude the black sheep’s genotype to be BB.

overview Punnett squares can be used to predict the genotypic and phenotypic ratios of two independently inherited genes. theory details In lesson 8A, you learned how to complete a monohybrid cross to predict the genetic outcomes for one trait. However, sometimes we may want to predict the genetic outcomes of two traits, and calculate the likelihood of different combinations of genes being expressed in offspring. To do this, we can use what are known as dihybrid crosses. In this section, we will focus on unlinked genes, which are found on separate chromosomes or far apart on the same chromosome. For example, assume we want to calculate the likelihood of an individual having both dimples and a cleft chin (Figure 1). For the gene responsible for dimples formation, D = dimples and d = no dimples, and for the gene responsible for cleft chin expression, C = cleft chin and c = no cleft chin. When writing the genotype of an individual for two separate genes, we write the genotype for one trait followed by the other, such that the genotype of a heterozygous individual for both traits is CcDd.

How to complete a dihybrid Punnett square 1 Assign letters to each allele 2 Draw a 4 × 4 grid 3 Write each of the father’s allele combinations above one column 4 Write each of the mother’s allele combinations beside one row 5 Complete the cross to determine the potential offspring genotypes. Ensure you write the dominant allele first in heterozygous offspring 6 Calculate the fractional proportions of each potential genotype by determining the frequency of each genotype then dividing it by the total number of squares. Multiply this fraction by 100 to determine the percentage of each genotype 7 Determine the phenotype of each offspring by looking at the genotypes. To calculate the fractional proportions of each phenotype, determine the frequency of each phenotype then divide it by the total number of squares. Multiply this fraction by 100 to determine the percentage of each phenotype. (Note: steps 3 and 4 are interchangeable, and it doesn’t matter which side the mother and father’s alleles are placed.) Table 1 Worked example of how t

As Figure 1 and Table 1 both demonstrate, there can be up to four different possible combinations of alleles in any one gamete. From here, phenotypic and genotypic proportions can be calculated, which will be considered in closer detail later in this lesson. Out of the 16 combinations: • nine potential offspring would have both a cleft chin and dimples • three would have a cleft chin but no dimples • three would have dimples but no cleft chin • one would have neither a cleft chin or dimples. This gives us a phenotypic ratio of 9 : 3 : 3 : 1. When considering the ratios from dihybrid crosses, an important tip to remember is that whenever you cross two heterozygous individuals, you will always finish with the same ratio of 9 : 3 : 3 : 1. Linked dihybrid crosses 2.1.11.2 overview The distance between two genes determines whether or not they are linked. Linked genes require a different format for dihybrid crosses. Theory details In the previous section, we learned how to complete dihybrid crosses with unlinked genes. Now we’ll take things one step further and examine how inheritance of linked genes can be predicted. Linked genes Genes that are located more closely together on the same chromosome are known as linked genes. This means that during meiosis, because they are on the same chromosome, they are inherited together and are not separated during independent assortment. Linked genes, however, can occasionally be separated through crossing over (Figure 3).

linked genes genes that are found close together on the same chromosome and are likely to be inherited together independent assortment  the random orientation of homologous chromosomes along the metaphase plate during metaphase I crossing over the exchange of genetic material between non-sister chromatids during prophase I of meiosis, resulting in new combinations of alleles in daughter cells

Linked genes can be found in fruit flies, Drosophila melanogaster (Figure 5). On their second chromosome are three linked genes that encode body colour, eye colour, and wing size. Their body colour can be grey (B) or black (b), their eye colour can be red (E) or brown (e), and their wing size can be normal (W) or vestigial (w). For unlinked genes, we can consistently predict offspring genotypes and phenotypes because all the genes sort independently. This means we get nice ratios like 1 : 1 : 1 : 1 when crossing a heterozygous individual with a homozygous recessive individual. For linked genes, however, we can’t assume two alleles on the same chromosome will end up in the same gamete. Because of crossing over, the two alleles may occasionally end up on different chromosomes and in different gametes. The chances of crossing over occurring are calculated by using map units (Figure 6). One map unit equates to a one per cent chance of crossing over and the offspring containing a recombinant chromosome. This means crosses with linked genes can result in strange ratios like 11.5 : 1 : 1 : 11.5 (Figure 7).

If we use the example of body colour and eye colour in Drosophila melanogaster, these genes are eight map units apart, meaning there is an eight per cent chance that crossing over will occur to create a recombinant chromosome. Alternatively, we can say 8 in 100 gametes will contain a recombinant chromosome. Thus, there is a 92 per cent chance that crossing over will not occur, resulting in a parental chromosome.

From this information, we can perform a linked dihybrid cross. For simplicity, usually these crosses are performed against a homozygous recessive individual (a test cross). Figure 8 shows a cross between a heterozygous male (Be/bE) and a homozygous recessive female (be/be). For the homozygous recessive female, only one gamete option is required for the cross, as even if crossing over occurs, the gametes will still have the parental allele combinations (be). There are four possible gametes (be, Be, bE, BE) for the heterozygous male as crossing over has created new combinations of alleles. For linked genes, genotypes are written using a different convention. You write the alleles on one chromosome first, then add a forward slash (/), then write the alleles on the second chromosome. Using the example in Figure 8, the genotype for the individual in the left square would be Be/be.

How to complete a linked dihybrid cross Punnett square Note: these steps are for a cross with a homozygous recessive individual only. 1 Assign letters to each allele 2 Determine each of the possible alleles in each gamete for the parents and the chances of this happening. Be sure to account for crossing over 3 Draw a 1 × 4 grid 4 Write each of the non-homozygous recessive individual’s allele combinations above one column and note which combinations are parental and which are recombinant 5 Write the homozygous recessive alleles next to the one row 6 Complete the cross to determine the potential offspring genotypes. Be sure to use the correct conventions for writing genotypes for linked genes 7 Write the percentage chance of this genotype occurring below each square Table 2 Worked example of completing a linked dihybrid cross Punnett square

Dihybrid crosses allow us to predict the potential genotypes of offspring for two genes. There are two methods for this depending on how close together the genes are – linked and unlinked dihybrid crosses. A common question you might come across is determining if a dihybrid cross involves linked or unlinked genes. A good clue for identifying linkage is seeing unusual genotypic ratios in offspring (i.e. a few genotypes that arise very rarely) or genotypes that were not present in the parents. Remember that if you did a dihybrid test cross between a heterozygote and a homozygous recessive individual, unlinked genes would result in a genotypic ratio of 1 : 1 : 1 : 1. This ratio would not occur if the genes were linked – it might be 1 : 1 : 0.1 : 0.1 or something unusual like that.

Pedigrees 2.1.9.1 Overview Pedigrees are a visual representation of how a trait is passed down through multiple generations. Theory details To analyse patterns of inheritance, geneticists use pedigree charts to characterise the inheritance of a trait over multiple generations. Figure 1 shows common symbols used in pedigree charts. While this is not an exhaustive list, for the purposes of VCE Biology, these are the symbols you need to know. Important conventions of pedigree charts include: • horizontal lines represent mating between two individuals • vertical lines represent the link between two generations • circles are females and squares are males • coloured shapes are affected individuals • uncoloured shapes are unaffected individuals.

When identifying autosomal traits, we can rule out sex-linked traits if the pedigree does not satisfy all the criteria of sex-linked inheritance. However, it is important to note that sex-linked inheritance cannot be confirmed from a pedigree as it is possible for autosomal traits to produce pedigree charts that suggest sex-linked inheritance. So, instead of saying that sex-linked inheritance is confirmed, we say that sex-linked inheritance is more likely, as the chances of autosomal traits producing a pedigree chart that looks like sex-linked inheritance are very low. For instance, it is entirely possible that the pedigree chart for Y-linked inheritance is caused by an autosomal dominant trait that, by complete chance, only affected males. More evidence, such as evidence obtained via gene sequencing and mapping, is required to conclusively identify sex-linked traits. 8C THEORY 397 HAEMOPHILIA Haemophilia is a blood disorder which affects an individual’s ability to form blood clots. This can lead to prolonged, internal bleeding, and deep bruises. Evaluate the pedigree to determine the inheritance pattern of this disease. Analysis of the pedigree: • Is this trait present in every generation? No. • For every affected female in the pedigree, is their father affected? Yes. • Therefore, it is likely this is a X-linked recessive disorder. However sex-linked inheritance cannot be confirmed as it is possible that the trait is an autosomal recessive disorder. I II III IV 1 2 1 2 3 4 1 2 3 4 5 6 7 1 2 3 4 Figure 7 Sample pedigree of haemophilia across four generations Steps to determine inheritance patterns When approaching a new pedigree, the steps in Figure 5 can help breakdown what type of inheritance pattern it is showing. For more complex pedigrees, you may need to refer back to the criteria in Table 1 to identify the inheritance pattern. Remember, a trait is not necessarily sex-linked just because it is more prominent in one sex. It must satisfy the criteria in Figure 5 or Table 1. Is the t

Determining genotypes From pedigree charts, you can determine the genotypes of certain individuals depending on the type of inheritance pattern the trait follows. In Figure 9, an autosomal dominant pedigree is shown. 1 2 I II III 1 2 3 4 5 6 1 2 3 4 5 6 Figure 9 Autosomal dominant pedigree across three generations Determining the genotype of I-1 and I-2: • As it is an autosomal trait, all individuals will have two alleles. • As it is a dominant trait, affected individuals may be homozygous or heterozygous. Therefore, they are not carriers. • Therefore, if an affected individual produces an unaffected child (meaning a child who possesses two unaffected alleles), that individual must be heterozygous (having passed on an unaffected allele to their child). • As I-1 is unaffected, he must be homozygous recessive. • As I-2 has produced an offspring – II-2 – who is unaffected (homozygous recessive), that means that they must be heterozygous at this gene locus. To make it easier to track genotypes in the pedigree chart, you might find it helpful to write all of an individual’s possible genotypes above their symbol. Theory summary Pedigree charts show the inheritance of a trait over many generations. We can analyse these charts to determine if a trait is dominant, recessive, sex-linked, or autosomal. Pedigrees! By tracking male baldness over generations, scientists have worked out that the main gene for male baldness is dominant on the X chromosome. So if your maternal grandfather is bald, there is at least a 50% chance that you are going to inherit the X-linked male baldness gene. However, don’t be too upset. While current science tells us there is no surefire way to regrow hair once it’s lost, ha